ModelDDEWhatIs

A Delay Differential Equation (or DDE) is a differential equation in which the derivative of the function in one instant of time depends on the value of the function in previous instants. In mathematical terms, {$$ x'(t) = f(t,x(t),x_{\tau})$$} where {$ x_{\tau} = \{ x(s) : \mbox{ for some } s \le t \}$}.

EJS can only solve DDEs with delays that are both discrete and constant. That is, equations of the form:
{$$ x'(t) = f(t,x(t),x(t-\tau_1),x(t-\tau_2),\ldots,x(t-\tau_n))$$}
where {$ \tau_1, \tau_2,\ldots, \tau_n$} remain constant while the DDE is being solved. (Though they can change value if the DDE is conveniently reinitialized. That is, you can associate the delays with a variable, but you need to reset the solver if you change the value of this variable.)